U.S. patent number 6,162,754 [Application Number 09/147,736] was granted by the patent office on 2000-12-19 for process for regenerating a catalyst contained within a bubble-column reactor with draft-tube and process for the production of a hydrocarbon.
This patent grant is currently assigned to Agip Petroli S.p.A., ENI S.p.A., Institut Francais du Petrole. Invention is credited to Dominique Casanave, Pierre Galtier, Cristina Maretto, Vincenzo Piccolo.
United States Patent |
6,162,754 |
Maretto , et al. |
December 19, 2000 |
Process for regenerating a catalyst contained within a
bubble-column reactor with draft-tube and process for the
production of a hydrocarbon
Abstract
Continuous process for the production of prevalently heavy
hydrocarbons starting from synthesis gas in the presence of a gas
phase, a liquid and a solid catalyst, the above process being
carried out using a bubble column, characterized in that the bubble
column internally has: (a) at least one draft-tube; (b) at least
one device for the inlet of the synthesis gas; (c) at least one
device for the inlet of the regenerating gas; (d) at least one
device for activating/interrupting the stream of regenerating gas;
(e) optional devices suitable for minimizing the mixing of the
synthesis gas with the regenerating gas.
Inventors: |
Maretto; Cristina (Padova,
IT), Piccolo; Vincenzo (Milan, IT),
Casanave; Dominique (Villeurbanne, FR), Galtier;
Pierre (Vienne, FR) |
Assignee: |
Agip Petroli S.p.A. (Rome,
IT)
ENI S.p.A. (Rome, IT)
Institut Francais du Petrole (Rueil-Malmaison,
FR)
|
Family
ID: |
11377442 |
Appl.
No.: |
09/147,736 |
Filed: |
March 18, 1999 |
PCT
Filed: |
June 20, 1998 |
PCT No.: |
PCT/EP98/03874 |
371
Date: |
March 18, 1999 |
102(e)
Date: |
March 18, 1999 |
PCT
Pub. No.: |
WO99/00191 |
PCT
Pub. Date: |
January 07, 1999 |
Foreign Application Priority Data
|
|
|
|
|
Jun 26, 1997 [IT] |
|
|
MI97A1509 |
|
Current U.S.
Class: |
502/31; 502/30;
518/709; 502/53; 518/706; 518/700 |
Current CPC
Class: |
B01J
8/226 (20130101); B01J 38/48 (20130101); C07C
1/049 (20130101); C10G 2/342 (20130101); B01J
2208/00132 (20130101); Y02P 20/584 (20151101); B01J
2219/00033 (20130101); B01J 2208/0084 (20130101) |
Current International
Class: |
B01J
38/00 (20060101); B01J 38/48 (20060101); C07C
1/04 (20060101); B01J 8/22 (20060101); B01J
8/20 (20060101); C07C 1/00 (20060101); C10G
2/00 (20060101); B01J 020/34 (); B01J 038/56 () |
Field of
Search: |
;502/30,31,53
;518/700,706,709 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3754993 |
August 1973 |
Oguchi et al. |
5252613 |
October 1993 |
Chang et al. |
5268344 |
December 1993 |
Pedrick et al. |
5288673 |
February 1994 |
Behrmann et al. |
5827902 |
October 1998 |
Maretto et al. |
|
Foreign Patent Documents
Primary Examiner: Griffin; Steven P.
Assistant Examiner: Ildebrando; Christina
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Claims
What is claimed is:
1. A continuous process for the production of a hydrocarbon,
comprising:
reacting a synthesis gas in the presence of a gas phase, a liquid
and a solid catalyst using a bubble column equipped with cooling
devices; and
regenerating a reversibly, partially deactivated catalyst in the
presence of a regenerating gas;
wherein the bubble column comprises:
(a) at least one draft tube, having a vertical cylinder and having
smaller dimensions than the bubble column, with both a lower and an
upper end open, completely immersed in the liquid phase containing
the solid catalyst in suspension;
(b) at least one device for an inlet for the synthesis gas;
(c) at least one device for an inlet of the regenerating gas
situated at a bottom of an interspace between the draft tube and an
internal wall of the bubble column;
(d) at least one device which activates or interrupts the stream of
the regenerating gas; and
(e) an optional device for minimizing a mixing of the synthesis gas
with the regenerating gas.
2. The process according to claim 1, wherein the draft tube is
positioned coaxially with respect to the bubble column.
3. The process according to claim 1, wherein the device (b) for the
inlet of the synthesis gas is a gas distributor.
4. The process according to claim 1, wherein the device (b) for the
inlet of the synthesis gas is situated at the bottom of the bubble
column.
5. The process according to claim 1, wherein the device (c) for the
inlet of the regenerating gas is a gas distributor.
6. The process according to claim 1, wherein the device (e) for
minimizing the mixing of the synthesis gas with the regenerating
gas is a deflector.
7. The process according to claim 1, wherein the device (e) for
minimizing the mixing of the synthesis gas with the regenerating
gas is assembled near the lower opening of the draft tube.
8. The process according to claim 1, wherein the lower end of the
draft tube (a) is situated just above a bottom of the bubble
column; and
wherein the upper end of the draft tube (a) is situated just below
a free surface of a solid-liquid suspension containing the solid
catalyst.
9. The process according to claim 1, wherein the hydrocarbon is a
heavy hydrocarbon, an alternative fuel, an octane enhancer, a
chemical, a chemical intermediate, or mixtures thereof.
10. A process for regenerating a reversibly, partially deactivated
solid catalyst, comprising:
regenerating a quantity of said partially deactivated solid
catalyst suspended in a liquid contained in an interspace between a
bubble column and a draft tube without interrupting a flow of a
synthesis gas by flushing a regeneration gas containing hydrogen
into said interspace;
balancing a hydrostatic head between said draft tube and said
interspace by adjusting the flow rate of said regeneration gas;
interrupting a feeding of said regeneration gas;
reestablishing a circulation of said liquid containing said solid
catalyst in suspension by means of said draft tube;
thereby substituting a regenerated catalyst in said interspace for
said partially deactivated solid catalyst from inside said draft
tube;
wherein said bubble column comprises:
(a) at least one draft tube, having a vertical cylinder and having
smaller dimensions than said bubble column, with both a lower and
an upper end open, completely immersed in said liquid containing a
solid catalyst in suspension;
(b) at least one device for an inlet for said synthesis gas;
(c) at least one device for an inlet of said regeneration gas;
(d) at least one device which activates or interrupts a stream of
said regeneration gas; and
(e) an optional device for minimizing a mixing of said synthesis
gas with said regeneration gas.
11. The process according to claim 10, wherein during said
regenerating said regeneration gas is flushed from a lower part of
said interspace between said bubble column and said draft tube.
12. The process according to claim 10, wherein the process is
repeated until there is a total regeneration of the catalyst in the
bubble column.
13. A process according to claim 10, wherein the catalyst is
selected from the group consisting of Group VIII metals.
14. A process according to claim 13, wherein the Group VIII metal
is nickel or cobalt.
Description
The present invention relates to a bubble-column reactor equipped
with a draft-tube which can be used in triphase slurry processes,
more specifically in the Fischer-Tropsch process.
The present invention also relates to a regeneration process of
catalysts partially and reversibly deactivated, which uses the
above reactor.
Slurry catalytic processes, i.e. operating in triphase systems
essentially consisting of a gas phase and a liquid phase in which
the solid catalyst is dispersed, particularly the Fischer-Tropsch
process, have the disadvantage of a more or less distinct
reversible deactivation of the initial catalytic activity of the
catalyst. This drawback is generally resolved by the regeneration
of the exhausted catalyst.
EP-A-590,882 describes a method for regenerating a catalyst for the
synthesis of hydrocarbons, containing Cobalt or Ruthenium, subject
to partial, reversible deactivation in a slurry synthesis process.
This process enables a recovery of at least 80% of the initial
activity of the catalyst.
The above method involves carrying out the regeneration of the
catalyst in-situ of the slurry reactor, by periodically stopping
the flow-rate of the process gas (synthesis gas) and sending a flow
of gas containing hydrogen and other inert gases, avoiding the
presence of components, such as for example carbon monoxide,
capable of reacting with the hydrogen.
The process described in EP-A-590,882 has the disadvantage however
of requiring a periodical interruption of the synthesis of
hydrocarbons to substitute the process gas with the gas containing
hydrogen.
U.S. Pat. No. 5,268,344 solves the problem by using a bubble column
and carrying out the regeneration of the catalyst inside one or
more draft-tubes situated inside the bubble-column reactor, the
fraction of the column section occupied by the draft-tubes being
preferably less than 10%. Unlike what is described in EP-A-590,882,
this solution does not involve the interruption of the synthesis
gas.
It is also known that in the field of reactions in triphase
systems, bubble-column reactors equipped with a draft-tube are
preferable to simple bubble-column reactors with respect to the
distribution of the solid phase in the triphase system.
A particular bubble-column configuration equipped with a draft-tube
has now been found which overcomes the above disadvantages.
In fact, the use of this particular bubble-column configuration
enables the regeneration of the partially deactivated catalyst to
be carried out in-situ (more specifically at the interspace between
reactor and draft-tube), thus avoiding the periodical interruption
of the feeding of the process gas.
In addition, the reactor of the present invention allows a better
homogenization of the phases with respect to the bubble-column
reactors used in the U.S. Pat. No. 5,268,344.
In accordance with this, the present invention relates to a
continuous process for the production of prevalently heavy
hydrocarbons, alternative fuels, octane enhancers, chemicals and
chemical intermediates starting from synthesis gas in the presence
of a gas phase, a liquid and a solid catalyst, the above process
being carried out using a bubble column equipped with cooling
devices and comprising the periodical internal regeneration of the
reversibly, partially deactivated catalyst, said regeneration being
carried out in the presence of a regenerating gas, characterized in
that the bubble column internally has:
(a) at least one draft-tube, consisting of a substantially vertical
cylinder, having smaller dimensions than the column, preferably
positioned co-axially with respect to the column, with both the
lower and upper ends open, completely immersed in the liquid phase
containing the solid in suspension;
(b) at least one device for the inlet of the synthesis gas,
preferably a gas distributor, preferably situated at the bottom of
the bubble-column;
(c) at least one device for the inlet of the regenerating gas,
preferably a gas distributor, preferably situated at the bottom of
the interspace between the draft-tube and the internal wall of the
reactor;
(d) at least one device which activates/interrupts the stream of
regenerating gas;
(e) optional devices suitable for minimizing the mixing of the
synthesis gas with the regenerating gas, preferably deflectors,
preferably assembled near the lower opening of the draft-tube.
The term "regenerating gas" means the gas, usually hydrogen
possibly diluted with inert gases, used for the
rejuvenation--regeneration of the reversibly deactivated solid
catalyst, preferably containing at least one metal of Group VIII,
preferably selected from cobalt and iron, preferably cobalt.
The configuration of the bubble-column reactor of the present
invention enables the catalyst to be regenerated without
interrupting the reagent gases stream.
FIG. 1 represents a non-limiting example of the embodiment of the
present invention. In order, the numbers refer to:
1--feeding line of the synthesis gas,
2--feeding line of the regenerating gas for the regeneration of the
catalyst,
3--discharge line of the gas products (prevalently light
hydrocarbons) and non-reacted components,
4--discharge line of the liquid products,
5--feeding line of the cooling fluid,
6--discharge line of the cooling fluid,
7--draft-tube,
8--cooling devices situated in the interspace between the draft and
reactor,
9--cooling devices situated inside the draft-tube,
10--device for the inlet of the synthesis gas,
11--device for the inlet of the regenerating gas,
12--deflectors,
13--valve for activating/interrupting the stream of regenerating
gas of the catalyst,
14--dispersion level (gas-liquid-solid).
In the diagram of FIG. 1 there are also arrows which indicate the
direction of the movement of the internal circulation of the
liquid, which is established by the draft-tube when the stream of
gas containing hydrogen is interrupted.
In compliance with what is described in FIG. 1, the bubble column
reactor of the present invention comprises internally a draft-tube
(7), substantially vertical, which uses the process gas as carrier.
This device is basically a vertical cylinder, with smaller
dimensions than the bubble-column reactor, which is co-axially
introduced inside the column, open at both ends and completely
immersed in the liquid containing the solid in suspension. This
allows the liquid and solid in suspension to circulate through the
cylindrical device and interspace outside the cylinder, if the
driving force due to the process gas entering the bottom of the
column overcomes the pressure drops. The dimensions of this device
(7) must be such that the lower end is preferably just: above the
bottom of the reactor, whereas the upper end is just below the free
surface of the solid-liquid suspension containing the gas.
The synthesis gas, comprising carbon monoxide and hydrogen, is
introduced into the bottom of the reactor by means of an
appropriate device, preferably a distributor (10). The geometry of
the distributor and the distance of the draft-tube from the bottom
of the column are adequately selected to allow the process gas to
flow inside the cylindrical device, thus avoiding preferential
routes in the interspace zone. The Fischer-Tropsch synthesis
reaction takes place inside the cylindrical device.
The regeneration of the catalyst is carried out with regenerating
gas, preferably hydrogen, at high temperatures and pressures,
corresponding to those adopted for the Fischer-Tropsch synthesis.
The hydrogen is fed as a gas stream; this stream can contain inert
gases, such as methane or other light hydrocarbons (C.sub.2
-C.sub.10). It is preferable for them not to contain carbon
monoxide or other components which can react with the hydrogen at
the operating temperature and pressure of the Fischer-Tropsch
synthesis.
As mentioned above, the above configuration of the bubble column
allows the deactivated catalyst to be regenerated in-situ.
A further object of the present invention relates to a process for
the regeneration in-situ of reversibly partially deactivated solid
catalysts, prevalently containing metals of group VIII, preferably
selected from cobalt and iron, the above method comprising the use
of a reactor as described in claim 1 and avoiding the interruption
of the synthesis gas during the above regeneration, which
comprises:
(i) a first regeneration phase of the catalyst, in which a
regenerating gas containing hydrogen is flushed into the interspace
between the reactor and draft-tube, preferably from the lower part
of the above interspace, for a time which is sufficient to
regenerate the quantity of exhausted catalyst suspended in the
liquid contained in the interspace, the flow-rate of the gas
containing hydrogen being such as to balance the hydrostatic head
between the draft zone and that of the interspace;
(ii) a second phase in which the feeding of the gas containing
hydrogen is interrupted and the circulation of the liquid
containing the solid in suspension is re-established by means of
the draft-tube; in this way the regenerated catalyst obtained in
phase (i) is substituted with the exhausted catalyst still present
inside the reactor;
(iii) repetition of phases (i) and (ii), preferably until the total
regeneration of the catalyst contained in the column reactor.
The term "regeneration of the catalyst" means the recovery of at
least 80% of the initial catalytic activity of the catalyst.
In step (i) it is preferable to minimize the circulation of the
liquid-solid suspension and gas between the cylindrical device and
the interspace; this is achieved by acting on the flow-rate of gas
containing hydrogen flushed into the interspace to balance the
hydrostatic head between the two regions.
Appropriately shaped deflectors can be installed in the lower
opening of the draft-tube to minimize the mixing of the two gas
streams, that of the process gas and that containing hydrogen for
regeneration.
In step (ii) the feeding of the regenerating gas is interrupted and
the circulation of the liquid containing the solid in suspension is
re-established by means of the draft-tube and process gas, whose
flow-rate basically remains unvaried, the latter depending
exclusively on the operating and process conditions
established.
As mentioned above, the regeneration of the catalyst takes place in
the interspace between the column and the cylindrical device, using
a regenerating gas, preferably introduced by means of suitable
distributors, preferably situated at the lower opening of the
annular interspace.
The establishment of a forced circulation of liquid containing the
solid in suspension between the draft-tube and interspace allows a
new charge of exhausted catalyst, suspended in the liquid
prevalently consisting of the hydrocarbons produced by the
synthesis process, to enter the interspace thus substituting the
suspension containing the regenerated catalyst.
The "regenerated catalyst", owing to the circulation established by
the draft-tube, leaves the interspace to enter the reaction zone
(inside the draft-tube) from the opening at the bottom, whereas the
charge of "exhausted catalyst" passes from the reaction zone to the
interspace, where the regeneration takes place, through the upper
opening.
When the volume of the interspace has been completely renewed, one
regeneration cycle is completed and the stream of regenerating gas
in the interspace is re-opened starting a new regeneration
cycle.
During both phase (i) and phase (ii), the Fischer-Tropsch reaction
takes place in continuous inside the cylindrical device, whose
volume represents the reaction volume, where the process gas is
flushed.
The regeneration cycle is started when the activity of the catalyst
deteriorates over a certain level, for example 50%, and is stopped
when the catalyst has recovered the desired catalytic activity,
preferably after recovering at least 80%, even more preferably at
least 90%, of the original catalytic activity.
When the regeneration cycle and renewal of the regenerated charge
is not carried out in the reactor of the present invention, the
column reactor operates with continuous internal circulation of the
liquid containing the solid in suspension due to the draft-tube
permanently installed inside the reactor.
As is known to experts in the field, the internal circulation
promotes the distribution of the solid in the suspension with the
liquid, which would otherwise only be achieved by means of the
bubbles of gas entering near the bottom of the column, thus making
the concentration profile of the catalyst more uniform.
Owing to the exothermicity of both the Fischer-Tropsch synthesis
reaction and the regeneration process, in order to maintain control
of the temperature and practically isothermal conditions, a
suitable cooling system is introduced into both the reaction and
regeneration sections, consisting for example of tube-bundles,
coils or other types of heat exchange surfaces immersed in the
suspension bulk (slurry). In the Fischer-Tropsch synthesis process
the temperature control is fundamental in that the temperature
directly affects the selectivity of the reaction; in addition, it
is important to preserve the catalyst from undesired overheating
which could damage it.
The internal regeneration of the catalyst preferably takes place
under the same conditions of temperature and pressure as the
Fischer-Tropsch synthesis reaction. In any case it is possible to
independently regulate the temperature both inside the reaction
zone and in the regeneration zone.
The conditions, particularly of temperature and pressure, for
synthesis processes of hydrocarbons are generally well known. The
temperatures can be between 150.degree. C. and 380.degree. C.,
preferably from 180.degree. C. to 350.degree. C., even more
preferably from 190.degree. C. to 300.degree. C. The pressures are
generally higher than about 0.5 MPa, preferably from 0.6 to 5 MPa,
more preferably from 1 to 4 MPa.
In the preferred embodiment of the present invention, i.e. in the
synthesis of hydrocarbons via reduction of CO, the solid particles
at least partly consist of particles of a catalyst selected from
those, well-known to experts in the field, normally used for
catalyzing this reaction. Any catalyst for the Fischer-Tropsch
synthesis, particularly those based on iron or cobalt, can be used
in the process of the present invention. Catalysts based on cobalt
are preferably used, in which the cobalt is present in a quantity
which is sufficient for being catalytically active for
Fischer-Tropsch. The concentrations of cobalt can normally be at
least 3% approximately, preferably from 5 to 45% by weight, more
preferably from 10 to 30% by weight, referring to the total weight
of the catalyst. The cobalt and possible promoters are dispersed in
a carrier, for example silica, alumina or titanium oxide. The
catalyst can contain other oxides, for example oxides of alkaline,
earth-alkaline, rare-earth metals. The catalyst can also contain
another metal which can be active as Fischer-Tropsch catalyst, for
example a metal of groups 6 and 8 of the periodic table of
elements, such as ruthenium, or which can be promoter, for example
molybdenum, rhenium, hafnium, zirconium, cerium or uranium. The
promoter metal(s) is usually present in a ratio, with respect to
the cobalt, of at least 0.05:1, preferably at least 0.1:1, even
more preferably from 0.1:1 to 1:1.
The above catalysts are generally in the form of fine powders
usually having an average diameter of between 10 and 700 .mu.m,
preferably from 10 to 200 .mu.m, even more preferably from 20 to
100 .mu.m. The above catalysts are used in the presence of a liquid
phase and a gas phase. In the case of Fischer-Tropsch synthesis,
the liquid phase can consist of any inert liquid, for example one
or more hydrocarbons having at least 5 carbon atoms per molecule.
Preferably, the liquid phase essentially consists of saturated
paraffins or olefinic polymers having a boiling point of more than
140.degree. C. approximately, preferably higher than 280.degree. C.
approx. In addition appropriate liquid media can consist of
paraffins produced by the Fischer-Tropsch reaction in the presence
of any catalyst, preferably having a boiling point higher than
350.degree. C. approx., preferably from 370 to 560.degree. C.
The loading of the solids, or the volume of the catalyst with
respect to the volume of suspension or diluent, can reach up to
50%, preferably from 5 to 40%.
In the case of Fischer-Tropsch, the feeding gas comprising carbon
monoxide and hydrogen, can be diluted with other gases, more often
up to a maximum of 30% by volume, preferably up to 20% by volume,
usually selected from nitrogen, methane, carbon dioxide.
As far as the ratio between hydrogen and carbon monoxide is
concerned, this can vary within a wide range. In the preferred
embodiment, it is between 1:1 and 3:1, even more preferably from
1.2:1 to 2.5:1.
The regeneration treatment increases the activity of synthesis
catalysts of hydrocarbons, reversibly and partially deactivated,
independently of the procedure with which they have been
prepared.
The following examples provide a better understanding of the
present invention.
EXAMPLES
Example 1 describes the conditions required for the regeneration of
the catalyst inside the bubble-column reactor with a draft-tube,
without interrupting the hydrocarbons synthesis process, with a
known geometry of the reactor and operating conditions at which the
process takes place.
In example 1 an industrial reactor is used with a diameter of 10 m,
having a draft-tube of 9.5 m in diameter, and the flow-rate of the
gas containing hydrogen to be flushed into the base of the annular
interspace, is calculated in relation to the flow-rate of the
process gas. Three cases are studied in example 1: 0.2, 0.3, 0.4 m
s.sup.-1 as surface velocity of the process gas referring to the
passage section of the draft-tube.
In example 2, the same conditions are maintained as in example 1,
varying however, instead of the flow-rate of the process gas, the
diameter of the draft-tube. The cases studied are 6.5, 8.5 and 9.5
m, whereas the surface velocity of the process gas, referring to
the section of the draft-tube, remains constant and equal to 0.3 m
s.sup.-1. As in example 1, the flow-rate of the gas containing
hydrogen, to be flushed into the base of the annular interspace, is
calculated this time in relation to the diameter of the
draft-tube.
Example 1
How to carry out the internal regeneration of the catalyst in a
bubble-column reactor without interrupting the stream of process
gas, with the synthesis of hydrocarbons in continuous.
I. Effect of the Flow-rate of the Process Gas
To ensure that the regeneration of the catalyst takes place without
interrupting the Fischer-Tropsch synthesis, inside the
bubble-column reactor equipped with a draft-tube, it is necessary
to avoid that:
(a) the stream of gas containing hydrogen, which is introduced into
the opening of the annular interspace, comes into contact with the
process gas containing CO, which reacts with the hydrogen, as
occurs in the synthesis process inside the draft-tube, preventing
the regeneration of the catalyst;
(b) the liquid containing the solid in suspension circulates
through the draft-tube and interspace, to be able to prevent mixing
of the volume of slurry in which the reaction takes place and the
volume of slurry in which the regeneration takes place, even if the
stream of gas containing hydrogen is periodically interrupted to
re-establish the forced circulation, due to the draft-tube, and to
renew the loading (or volume) of slurry to be regenerated inside
the annular interspace.
To satisfy the above items, in addition to suitable distribution
systems of the process gas and gas containing hydrogen, the
circulation of the liquid containing the catalyst in suspension
must be minimized; to do this the hydrostatic head (which is the
driving force of the liquid circulation) between the interspace and
the draft-tube must tend towards zero:
wherein:
.DELTA.P.sub.H =hydrostatic head between the interspace and the
draft-tube, Pa;
.epsilon..sub.d =gas holdup in the draft-tube;
.epsilon..sub.a =gas holdup in the interspace;
.rho..sub.G =density of the gaseous phase, kg m.sup.-3 ;
.rho..sub.SL =density of the slurry phase, kg m.sup.-3 ;
g=gravity acceleration, m s.sup.-2 ;
H=height of the free surface of the dispersion with respect to the
bottom of the column, m.
In the balance (I) it is assumed that the average concentration of
solid is the same in both the draft-tube and interspace, and also
that the density of the process gas is comparable to that of the
gas containing hydrogen for the regeneration of the catalyst.
In order to minimize the hydrostatic head, considering that the
density of the slurry is at least an order of magnitude higher than
that of the gas and that therefore their difference is always a
finite value, there must be the same gas holdup in both the
draft-tube and the interspace:
The above equation (II), when the reaction conditions, the geometry
of the bubble-column reactor comprising the draft-tube and the
flow-rate of process gas are established, can only be obtained with
a specific flow-rate of gas containing hydrogen flushed into the
interspace.
To describe the gas holdup in the draft-tube and interspace, a
hydrodynamic model from literature was adopted (Krishna et al.,
A.I.Ch.E. Journal Vol. 43, 1997, pages 311-316) valid for a
bubble-column in the presence of a gas-liquid-solid system with the
slurry phase under "batch" conditions, which estimates the gas
holdup in relation to the properties of the system, the diameter of
the column and the superficial velocity of the gas. With respect to
the annular region of the interspace, this was compared to a column
with a diameter equal to the corresponding hydraulic diameter.
The hydrodynamic model from literature is applied by referring to a
bubble-column reactor operating in the heterogeneous flow regime,
which is typical of industrial-sized reactors, as is known to
skilled person. The heterogeneous regime can be represented by
means of a generalized two-phase model, in which one phase called
"diluted" consists of the fraction of gas which flows through the
reactor in the form of large bubbles. The second one ("dense"
phase) can be represented by the liquid phase in which the
particles of solid are suspended and the remaining fraction of gas
in the form of small finely dispersed bubbles. The large bubbles,
having a higher rise velocity than the small bubbles, can be
essentially considered as being in plug flow. The dense phase,
consisting of the liquid, suspended solid and small finely
dispersed bubbles, has a certain degree of backmixing which depends
on the operating conditions of the process and the diameter of the
column. The hydrodynamic model from literature, which is based on a
large number of experimental results, also assumes that the
dependency of the gas holdup on the diameter of the column is valid
up to a column diameter of 1 m, for higher diameters this influence
being negligible. This can be explained by the fact that with a
diameter of more than 1 m the bubbles of gas in the slurry bulk are
no longer affected by the phenomenum known as "wall effect".
Considering an industrial-sized bubble-column reactor, with a
diameter of 10 m, in which the height of the slurry dispersion
containing the gas is 30 m, inside of which is a draft-tube with a
diameter of 9.5 m and a height of 29.8 m, situated at a distance of
10 cm above the bottom of the column in a co-axial position, the
flow-rate of gas containing H.sub.2 which satisfies balances (I)
and (II), was examined in relation to the flow-rate of process gas
to be adopted for the synthesis of hydrocarbons. The results, for
the three cases in which the surface velocity of the process gas,
referring to the section of the draft-tube, is equal to 0.2, 0.3
and 0.4 m/s, are indicated in table 1. In the above table, the gas
holdup is also represented, which is the same, by definition, in
the draft-tube and in the interspace when the catalyst is being
regenerated. Table 2 on the other hand shows the flow-rates of
liquid, containing the solid in suspension, which circulates
through the interspace and draft-tube when the stream of gas
containing hydrogen in the interspace, is interrupted, for the same
cases described in table 1. These flow-rates of liquid were
obtained by determining the actual velocity of the slurry (liquid
with solid in suspension) in the draft-tube and interspace which
satisfy the energy balance:
where .DELTA.P.sub.H is the hydrostatic head between the interspace
and the draft-tube, in Pa, whereas .DELTA.P.sub.LOSS refers to the
total pressure drops of the bubble-column reactor with a
draft-tube, which are obtained from the sum of the pressure drops
by friction in the interspace, in the draft-tube and at the top and
bottom of the draft-tube, where sudden section restrictions or
enlargements and inversions of the slurry flow direction, take
place.
The reaction conditions for all the cases are: 230.degree. C. and
30 bars, the concentration of catalyst is 35% by volume, the
density of the slurry 906 kg m.sup.-3.
TABLE 1 ______________________________________ regeneration phase
U.sub.d (m s.sup.-1) .epsilon..sub.d, .epsilon..sub.a U.sub.a (m
s.sup.-1) ______________________________________ 0.2 0.189 0.16 0.3
0.219 0.24 0.4 0.245 0.32
______________________________________
TABLE 2 ______________________________________ internal circulation
phase of the slurry U.sub.d (m s.sup.-1) Q.sub.L (m.sup.3 s.sup.-1)
______________________________________ 0.2 17 0.3 19.5 0.4 21.5
______________________________________
Example 2
How to carry out the internal regeneration of the catalyst in a
bubble-column reactor without interrupting the stream of process
gas, with the synthesis of hydrocarbons in continuous.
II. Effect of the Diameter of the Draft-tube
In this example the same assumptions are maintained as for example
1, but instead of varying the superficial velocity of the process
gas, the diameter of the draft-tube is varied. As in the previous
example, the diameter of the industrial-sized column is 10 m, the
height of the slurry dispersion containing the gas is 30 m, the
height of the draft-tube is kept constant and is equal to 29.8 m,
also the distance between the lower end of the draft-tube and the
bottom of the column is assumed as being constant and equal to 10
cm. The surface velocity of the gas with respect to the free
section of the passage of the draft-tube is set at 0.3 m s.sup.-1,
whereas the operating pressure and temperature of the synthesis
process of hydrocarbons are, as in the previous example, 30 bars
and 230.degree. C.
The flow-rate of gas containing hydrogen which satisfies balances
(I) and (II) of example 1 was examined in relation to the diameter
of the draft-tube, D.sub.d. The results, for three different
diameter values of the draft-tube: 6.5, 8.5 and 9.5 m, are
indicated in table 3, together with the area fraction occupied by
the interspace with respect to the total area of the column (A
%).
TABLE 3 ______________________________________ regeneration phase
D.sub.d (m) A % U.sub.a (m s.sup.-1)
______________________________________ 6.5 58% 0.3 8.5 28% 0.3 9.5
10% 0.24 ______________________________________
As can be seen from the results of table 3, for the cases of a
diameter of the draft-tube of 6.5 and 8.5 m, the superficial
velocity which the gas containing hydrogen must have to satisfy
balances (I) and (II) is the same as the process gas. The reason
for this is that for both cases the hydraulic diameter relating to
the interspace is greater than 1 m, therefore balance (II), for the
assumptions indicated in example 1, proves to be independent of the
diameter and depends exclusively on the gas velocity: as the
correlations which describe the gas holdup in the interspace and
the draft-tube are the same, balance (II) is only satisfied for the
same gas-liquid-solid system when the superficial velocities of the
two gases are the same.
When the stream of gas containing hydrogen is interrupted and the
internal circulation of the liquid containing the solid in
suspension is restarted, the flow-rates of circulating slurry which
are obtained, for the same cases as table 3, are indicated in table
4.
TABLE 4 ______________________________________ internal circulation
phase of the slurry D.sub.d (m) Q.sub.L (m.sup.3 s.sup.-1)
______________________________________ 6.5 13.8 8.5 18 9.5 19.5
______________________________________
As can be observed from the data of table 4, the increase in
diameter of the draft-tube increases the circulation of the slurry,
similarly to what occurs when the flow-rate of the synthesis gas is
increased maintaining the size of the draft-tube constant (see
table 2).
TABLE 5 ______________________________________ effect of the
diameter of the draft-tube on the circulation of the slurry D.sub.d
(m) Q.sub.L (m.sup.3 s.sup.-1)
______________________________________ 9.7 16.6 9.8 9.7 9.9 2.1
9.95 0.11 ______________________________________
To maximize the reaction volume with respect to the regeneration
volume, the section of the interspace must be reduced by
increasing, with the same external diameter of the reactor, the
diameter of the draft-tube.
However, if the diameter of the draft-tube is increased over a
certain limit value, there is a sudden drop in the circulation
flow-rate of the slurry (see table 5). This means that the draft
effect induced by the presence of the draft-tube is diminished,
whereas a certain amount of backmixing (undesired phenomenum) takes
place inside the reaction volume. In table 5 it can be seen that,
to have enough slurry recirculation, a draft-tube with a diameter
less than 9.8 meters must be selected.
The conditions of table 5 are the same as table 4.
* * * * *